Zimbabwe: Land-use in Dry Tropical Savannas
Soils and soil erosion
Soil erosion involves the detachment of soil particles from larger aggregates (or peds) and the removal of the particles by flowing water and wind. In the climatic conditions which prevail throughout most of Zimbabwe fluvial erosion is dominant, that is the effects of rain splash and run-off, both surface and subsurface. Soil erosion is an extremely complex phenomenon varying both spatially and temporally. Energy forces are those which determine how much energy is applied in the process of detaching and transporting soil particles. Resistive forces are those which help overcome the applied energy forces and relate mainly to soil properties. For example, where soils have a high organic matter content and have stable aggregates they are able to withstand the effects of raindrop impact more readily than soils with low organic matter content and unstable aggregates. Protective forces are those which neutralize energy forces. For example, a good plant cover will intercept raindrops and dissipate their energy before they reach the ground. Protective forces, however, include a variety of human factors such as soil conservation practices which may increase or reduce erosion depending on their presence or absence or in some cases, inappropriate use.
Rates of soil formation in Zimbabwe are very slow (e.g. 400 kgs/ha/year), whereas rates of soil erosion are very much greater; estimates for average soil losses on crop lands and grazing areas on commercial farms are 5 tonnes/ha/year and 3 tonnes/ha/year respectively; the equivalent average for communal lands are 50 and 75 tonnes/ha/year. These rates have been generally accepted for a long time but more recent observations indicate that these rates may be largely exaggerated. These rates were the result of a survey carried out on a specific single soil type in one ecological region, thus the replicability of these findings across a country, with high variability, in terms of the erosive and stabilizing factors, is questionable.
The consequences of this erosion are seen in general declines in crop yields and very high rates of siltation of reservoirs, especially of the smaller dams used for rural water supplies. Small dams are likely to fill with sediments within 15 years of construction and even the larger irrigation schemes are being affected adversely by siltation problems. It has been also estimated that in some areas the cultivation of maize may only be possible for another 15 years before soils become too shallow for crops growth, sorghum cultivation may be impossible within 30 years.
Soil erosion is clearly an important problem in the study area especially within the communal lands. It is essential that conservation measures be considered a key part of development strategies in these areas rather than being treated as a token appendage. It should be noted however that the parameters for defining soil erosion and land degradation are being re-visited. It is slowly being accepted that the definition of land degradation are flawed. For instance, the communal lands are located in cropping marginal areas where the ecology is inherently fragile. On the other hand, the commercial farming areas are on prime land, with very stable clayey soils. To then make a comparison of rates of soil erosion and land degradation between these two agricultural sectors on the premise that degradation or lack of it, is the result of either good or bad husbandry makes little sense.
The two most important factors which contributes to the statistical variation in erosion are soil type soil and population density. There is a direct positive correlation between increases in the extent of eroded terrain, soil type and increases in population density. This relationship is especially valid for the Communal Lands, which are located in the main in agro-ecological regions , III, IV and V.
Areas with negligible soil erosion are located mainly in south east, west and
extreme north of Zimbabwe (Fig 1) * here Fig 5 from the list should be included.
Much of this land is located within national parks and wild life areas.
The carrying capacity concept
A crucial concept in assessing the presence and consequences of pressure on the land is the concept of carrying capacities. Any area of land will support in perpetuity only a limited number of people and if this limit is exceeded without a compensating change in the system of land usage, then a cycle of degenerative changes is set in motion, which must result in deterioration or destruction of the land. A decline in resources status, in turn has adverse effects on living standards. As poverty becomes worse, so the ability to effect changes in land use to reverse the cycle of degenerative changes is reduced. In this way a vicious cycle of poverty and land degradation is established.
The carrying capacity concept is a rather illusive one. While it is generally agreed that there is a limit to which a finite unit of resources can sustain human and/or animal livelihood, arriving at ungeneralized-generalized carrying capacity is difficult. Carrying capacity changes with ecological regions, seasonal and within season variations. Some projections into the carrying capacity break points of some communal lands have long been exceeded, bringing into question the soundness of the parameters and methods used for such assessments.